1. Field of the Invention
The invention pertains to the field of cryorefrigeration. More particularly, the invention pertains to an integrated component for a pulse tube cryorefrigerator.
2. Description of the Related Art
Typical closed-cycle expansion cryogenic refrigerators include the Stirling, Gifford-McMahon and pulse tube types, all of which provide cooling through the alternating compression and expansion of a cryogen, with a consequent reduction of its temperature. Stirling and Gifford-McMahon regenerative refrigerators use displacers to move a cryogen (usually helium) through their regenerators, exhaust the heat in the return gas to the compressor package. The noise and vibration induced by the displacer creates problems, and the wear of the seals on the displacer require periodic maintenance and replacement.
Therefore, it is highly desirable to invent cryorefrigeration devices that generate less vibration and less acoustic noise than prior art cryorefrigerators. It is also desirable to decrease the number of moving parts used in cryorefrigeration devices and to significantly increase the required maintenance intervals.
Pulse tube refrigerators are a known alternative to the Stirling and Gifford-McMahon types, which do not use a mechanical displacer.
A pulse tube is essentially an adiabatic space wherein the temperature of the working fluid is stratified, such that one end of the tube is warmer than the other. A pulse tube refrigerator operates by cyclically compressing and expanding a cryogen in conjunction with its movement through heat exchangers. Heat is removed from the system upon the expansion of the cryogen in the gas phase.
Prior art single-stage valved pulse tube cryorefrigerators generally include a pulse tube, a rotary valve to generate the oscillating compression-expansion cycle, a reservoir to contain the expanding cryogen gas, orifices for the movement and phasing of the gas between the reservoir or buffer volume and the rest of the system, and a regenerator for absorbing heat temporarily and reversibly. Single stage pulse tube cryorefrigerators are generally capable of reaching temperatures above 20xc2x0 K., and achieving lower temperatures has in the past required staging of the pulse tubes. U.S. Pat. No. 3,237,421 to Gifford and other prior art publications disclose multistage pulse tube cryorefrigerators.
Prior art two-stage pulse tube cryorefrigerators generally include, in addition to the foregoing components, a first-stage pulse tube, a first-stage regenerator, a second-stage pulse tube, a second-stage regenerator and first and second cooling stages.
Although an improvement over mechanical displacement devices, prior art pulse tube cryorefrigerators were ungainly arrangements of separate components, which leads to inefficiency and difficulty in manufacture and maintenance.
Pulse tube coolers can be employed in a wide variety of applications from civilian to government to military. Most of the applications below are dependent on the availability of a cheap cryocooler with a long lifexe2x80x94long life is a unique advantage of the pulse tube cooler.
Sensors: Infrared; atmospheric studies, thermal losses, pollution monitoring, process monitoring, night vision, missile guidance, missile surveillance, Gamma-ray, monitor nuclear activity
Semiconductors in computers: (large speed gain at small cost penalty, temperatures around 100xe2x80x94200 K.)
Hi-Tc superconductors: Cellular phone base stations (more channels, temperatures under 80 K.), High speed computers, SQUID magnetometers, heart and brain studies
Magnets: maglev trains, mine sweeping
Cryopumps for the semiconductor industry
Cryogenic catheters, Cryosurgery
Liquefaction of gases: Helium, Hydrogen, Neon, Nitrogen, Argon Oxygen, Natural Gas, etc.xe2x80x94remote wells or peak shaving (providing extra gas at peak loads to minimize steady pipeline capacity) or for fleet vehicles
Perhaps the application of cryorefrigeration which is most familiar to the public is its use in Magnetic Resonance Imaging (hereinafter xe2x80x9cMRxe2x80x9d). MRI is an imaging technique used widely within the medical field to produce high quality images of the inside of a human body.
Generally, the most expensive component of a MRI system is the imaging magnet, which is typically an electromagnet made from a superconducting material. When cooled to a temperature near absolute zero (i.e., xe2x88x92273.15xc2x0 C. or 0xc2x0 K.), the superconducting wire in the magnet""s coil has an electrical resistance approaching zero. Therefore, MRI imaging magnets are usually maintained at a temperature of 4.2xc2x0 K. using liquid helium.
Typically, the main superconducting coils of a MRI imaging magnet are enclosed in a pressure vessel contained within an evacuated vessel (i.e., Dewar vessel), and superconducting temperatures are obtained by boiling a liquid cryogen, such as liquid helium, within the pressure vessel. Because distribution, storage and handling of liquid helium is difficult and costly, mechanical displacement cryorefrigerators, such as the Gifford-McMahon type, typically are used to condense and recycle the helium gas generated by boiling the liquid cryogen.
One problem associated with cryorefrigerators using displacers is that the motion of the displacer creates a series of repetitive knocking sounds and mechanical vibrations, which become especially rapid as the magnet in the MRI is cycled on and off to generate the magnetic field gradients that are used to collect information regarding the molecular structure of a patient""s body. The MRI equipment thus generates high acoustic noise levels, and also vibrates. Because of the volume of this noise, it is recommended that patients undergoing MRI use hearing protection devices. In fact, some MRI imaging sites even go to such lengths as to provide an airplane-like audio headphone system for their patients, in order to protect their hearing and mask the acoustic noise, which may agitate or frighten the patient.
The present invention is a component for use in pulse tube cryorefrigerators which integrates one or more of the reservoirs (buffer volumes) as well as the housing for the rotary valve and valve plate and drive motor into a convenient, unified assembly. Other components required by the pulse-tube refrigerators, such as the heat sink, orifices, phase shifting valves, connecting tubing, etc., may also be integrated into the buffer volume/valve/motor housing within the teachings of the invention.
Cryorefrigerators using the novel component have increased efficiency, reduced manufacturing cost, and increased compatibility with varied cryostats due to the compactness of the component.